In the cooperation with 6 hospitals in Yamagata Prefecture, a prospective survey of delayd adverse reactions (DARs) to nonionic contrast media (CM) was conducted in 16,700 patients who underwent CT and angiography from October 1994 to December 1995. Out of 16,700 patients, 11,783 returned the questionnaire. DARs were defined as those that occurred more than 1 hour but within 7 days after completion of the examination. The overall rate of DARs was 9.3%. Prevalence of skin rash was 2.9%. Statistically higher incidence of skin rash was noted in Isovist group than the other CM (Iopamidol, Omnipaque, Iomeron, Optiray). Picked up risk factors for DARs were as follows. (1) Patients without previous CM study. (2) Patients with previous adverse reactions to CM.(3) Patients with a history of allergy. (4) Patients with higher serum creatinine level than 2.0 mg.Out of patients with skin reactions, 44 consulted dermatologists. Of these, 27 received allergy test (patch and intradermal test). Patients with skin reactions to Iopamiron and Omnipaque were positive for intradermal test to the same CM at the rate of 75% and 30% respectively, and some of them showed positive cross reactions to the other CM.Patients with skin reactions to Isovist were positive for intradermal test to it at the rate of 67%, but there was no positive cross reactions to the other CM.Further investigation might be necessary for clarifying the significance of the phenomenon in allergy tests.